Comment 12 for bug 321578

Eventually, I've found that it was PowerPlay and not ACPI that was causing this issue....PowerPlay is used by ATI card to reduce consumption of graphic cards....and with open source drivers, it's not implemented yet...so I've installed proprietary drivers and now, the fan is running at normal speed and the general temperature is even cooler...
Something positive : powerplay will be soon available with latest branch of open source drivers ;)